Multijunction solar cells having a graded-index reflector structure

Abstract

A multijunction solar cells that include one or more graded-index reflector structures disposed beneath a base layer of one or more solar subcells. The graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through a solar cell above the graded-index reflector structure is reflected back into the solar subcell by the graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the solar cell above the graded-index reflector structure is transmitted through the graded-index reflector structure to layers disposed beneath the graded-index reflector structure. The second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range.

The invention claimed is: 
     
       1. A multijunction solar cell comprising:
 an upper solar subcell having an emitter layer and a base layer forming a photoelectric junction; 
 a first graded-index reflector structure disposed beneath the base layer of the upper solar subcell; wherein the first graded-index reflector structure comprises a first plurality of pairs of alternating layers of Al x Ga (1-x) As and a different semiconductor material, wherein 0<x<1 and a mole fraction of aluminum is increased for each of the Al x Ga (1-x) As layers in the first plurality of pairs of alternating layers as the distance between the alternating layers and the upper solar subcell increases; and 
 a lower solar subcell disposed beneath the first graded-index reflector structure; wherein the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction; 
 wherein the first graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the upper solar subcell is reflected back into the upper solar subcell by the first graded-index reflector structure; (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the upper solar subcell is transmitted through the first graded-index reflector structure to layers disposed beneath the first graded-index reflector structure, wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range. 
 
     
     
       2. The multijunction solar cell of  claim 1  further comprising a second graded-index reflector structure disposed beneath the base layer of the lower solar subcell;
 wherein the second graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the lower solar subcell is reflected back into the lower solar subcell by the second graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the lower solar subcell is transmitted through the second graded-index reflector structure to layers disposed beneath the second graded-index reflector structure, wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range. 
 
     
     
       3. The multijunction solar cell of  claim 1 , wherein the different semiconductor material is Al 0.15 Ga 0.85 As. 
     
     
       4. The multijunction solar cell of  claim 1 , wherein the upper subcell is composed (In)GaAs with a band gap of 1.41 eV, and the reflector structure includes a layer of Al 0.15 Ga 0.85 As with a band gap of 1.61 eV. 
     
     
       5. The multijunction solar cell of  claim 1 , wherein the multijunction solar cell is an upright multijunction solar cell, or an upright metamorphic solar cell, or an inverted metamorphic multijunction solar cell. 
     
     
       6. The multijunction solar cell of  claim 1 , wherein the multijunction solar cell is a III-V compound semiconductor multijunction solar cell and wherein the increase in mole fraction of aluminum in the Al x Ga (1-x) As layer in the first plurality of pairs of alternating layers ranges from Al 0.2 Ga 0.8 As to Al 0.9 Ga 0.1 As. 
     
     
       7. The multijunction solar cell of  claim 1 , wherein the first graded-index reflector structure further comprises:
 a second plurality of pairs of alternating layers of Al x Ga (1-x) As and the different semiconductor material wherein a mole fraction of aluminum for each of the Al x Ga (1-x) As layers in the second plurality of pairs of alternating layers is repeated; and 
 a third plurality of pairs of alternating layers of Al x Ga (1-x) As and the different semiconductor material wherein a mole fraction of aluminum for each of the Al x Ga (1-x) As layers is decreased in the third plurality of pairs of alternating layers as the distance between the alternating layers in the third plurality of pairs of alternating layers and the upper solar subcell increases. 
 
     
     
       8. The multijunction solar cell of  claim 7 , wherein the first plurality of pairs of alternating layers comprises Al x Ga (1-x) As/Al 0.15 Ga 0.85 As, wherein x is gradually increased from 0.2 to 0.9, such that the alternating pairs of Al x Ga (1-x) As/Al 0.15 Ga 0.85 As increase from Al 0.2 Ga 0.8 As/Al 0.15 Ga 0.85 As up to Al 0.9 Ga 0.1 As/Al 0.15 Ga 0.85 As, the second plurality of pairs of alternating layers comprises Al 0.9 Ga 0.1 As/Al 0.15 Ga 0.85 As wherein x remains constant, and the third plurality of pairs of alternating layers comprises Al x Ga (1-x) As/Al 0.15 Ga 0.85 As wherein x is gradually decreased from 0.9 to 0.2, such that the alternating pairs of Al x Ga (1-x) As/Al 0.15 Ga 0.85 As decrease from Al 0.9 Ga 0.1 As/Al 0.15 Ga 0.85 As down to Al 0.2 Ga 0.8 As/Al 0.15 Ga 0.85 As. 
     
     
       9. The multijunction solar cell of  claim 8 , wherein the upper subcell is (In)GaAs with a band gap of 1.41 eV, and a Al 0.15 Ga 0.85 As layer with a bandgap energy of 1.61 eV as the lowest band gap material in the first graded-index reflector structure, and the multijunction solar cell is an upright multijunction solar cell. 
     
     
       10. The multijunction solar cell of  claim 1 , wherein the reflector structure has a central wavelength primary reflectance peak and a substantial reduction in side lobe peaks which would result in reflection losses around the wavelength of such side lobe peaks. 
     
     
       11. A multijunction solar cell comprising:
 an upper solar subcell having an emitter layer and a base layer forming a photoelectric junction; 
 a first graded-index reflector structure disposed beneath the base layer of the upper solar subcell, wherein the first graded-index reflector structure comprises a plurality of pairs of alternating layers of Al x Ga (1-x) As and a different semiconductor material, wherein 0<x<1; 
 wherein for a first plurality of pairs of alternating layers, the difference in the indices of refraction between alternating layers increases as the distance between the alternating layers and the upper solar subcell increases; 
 wherein for a second plurality of pairs of alternating layers, the difference in the indices of refraction between alternating layers decreases as the distance between the alternating layers and the upper solar subcell increases, and 
 wherein the first plurality of pairs of alternating layers is spaced apart from the second plurality of pairs of alternating layers by a plurality of central alternating layers wherein the difference in the indices of refraction between central alternating layers remains constant; and 
 a lower solar subcell disposed beneath the first graded-index reflector structure, wherein the lower solar subcell has an emitter layer and a base layer forming a photoelectric junction; 
 wherein the first graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the upper solar subcell is reflected back into the upper solar subcell by the first graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the upper solar subcell is transmitted through the first graded-index reflector structure to layers disposed beneath the first graded-index reflector structure; wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range. 
 
     
     
       12. The multijunction solar cell of  claim 11 , further comprising a second graded-index reflector structure disposed beneath the base layer of the lower solar subcell;
 wherein the second graded-index reflector structure is constructed such that (i) at least a portion of light of a first spectral wavelength range that enters and passes through the lower solar subcell is reflected back into the lower solar subcell by the second graded-index reflector structure; and (ii) at least a portion of light of a second spectral wavelength range that enters and passes through the lower solar subcell is transmitted through the second graded-index reflector structure to layers disposed beneath the second graded-index reflector structure, wherein the second spectral wavelength range is composed of greater wavelengths than the wavelengths of the first spectral wavelength range. 
 
     
     
       13. The multijunction solar cell of  claim 11 , wherein the different semiconductor material is Al 0.15 Ga 0.85 As. 
     
     
       14. The multijunction solar cell of  claim 11 , wherein the upper subcell is (In)GaAs with a band gap of 1.41 eV, and the reflector structure includes a layer of Al 0.15 Ga 0.85 As with a band gap of 1.61 eV. 
     
     
       15. The multijunction solar cell of  claim 11 , wherein the multijunction solar cell is an upright multijunction solar cell, an upright metamorphic multijunction solar cell, or an inverted metamorphic multijunction solar cell.